GASIFICATION OF REFUSE-DERIVED FUEL (RDF)

Transcription

1 37 GASIFICATION OF REFUSE-DERIVED FUEL (RDF) Juma ayday Insttute of Chemcal and Envonmental Engneeng, Slovak Unvesty of Technology n Batslava Radlnskeho 9, Batslava, Slovaka e-mal: juma.hayday@stuba.sk Abstact In ths wok, the gasfcaton of a facton of muncpal sold waste, MSW, geneally sepaated fom noganc mateals and bodegadable components, the so-called efuse-deved fuel (RDF), was studed usng mateal chaactesaton methods, and the modellng of an ndustal scale pocess was pesented. The composton of RDF was detemned by the sepaaton of a epesentatve sample nto ts basc components (pape, fols, had plastcs, textles). All RDF components as well as a epesentatve mxed sample of the RDF wee studed usng a themogavmetc analyss (TGA), elemental analyss and bomb calomety to detemne the poxmate and elemental compostons, and a hghe heatng value. An ndustal scale gasfcaton pocess was studed by mathematcal modellng and compute smulatons. All technques, gasfcaton wth a, gasfcaton wth oxygen, and gasfcaton wth both oxygen and steam wee nvestgated unde dffeent condtons. The RDF conveson of 100 % was acheved by the gasfcaton wth a at the a to RDF mass ato of 3.2. The gas heatng value was 4.4 MJ/Nm 3. The gasfcaton of RDF usng oxygen enables the poducton of gas wth the heatng value of aound 10 MJ/Nm 3 at the oxygen to RDF mass ato of By nceasng the steam to the RDF mass ato, the contents of 2 and CO 2 nceased, whle the content of CO, eacto tempeatue and the gas heatng value deceased. 1 INTRODUCTION Many countes, ncludng Slovaka, stll landfll the majoty of the muncpal sold waste (MSW). Such landfllng esults not only n sgnfcant envonmental challenges, but also n wastng valuable esouces. MSW epesents a valuable souce of mateal and enegy. oweve, the heteogenety and composton vaatons ae mpotant challenges fo mechancal ecyclng of MSW. On the othe hand, feedstock ecyclng methods such as pyolyss and gasfcaton have a bg potental to be used fo ecovey of valuable chemcals and enegy fom MSW. The facton of MSW geneally sepaated fom noganc mateals and bodegadable components s called efuse-deved fuel (RDF). The gasfcaton of RDF esults n the poducton of a combustble gas contanng manly 2, CO, CO 2, and N 2 f a s used as the gasfcaton agent. Some methane, lght hydocabons, 2 S, N 3, and ta ae also pesent n the gas [1]. A numbe of powe geneaton plants based on gasfcaton technology ae n opeaton woldwde. A lst of opeatng and planed gasfcaton plants has been povded by gman and Van de Bugt [2]. The modellng of gasfcaton enables pedctng optmal pocess condtons and educng the numbe of expements n the pocesses of desgn and opeaton. A smple equlbum mathematcal model based on the mnmsaton of Gbbs fee enegy was used by some authos to model the gasfcaton of RDF. Mateazz et al. [3] used the equlbum model to evaluate a two-stage waste gasfcaton system. It esults fom the wok that except fo 2 the pedcted composton of gas s n good coheence wth the expemental data. The equlbum model was also appled by Baba et al. fo RDF gasfcaton [4], and Jaungthammachote and Dutta [5] fo the gasfcaton of chacoal and coconut shell. The authos found that the model data s n good ageement wth the expemental one. oweve, L et al. [6] appled an equlbum model fo coal gasfcaton and declaed that ths model has some lmtatons n pedctng gas composton. Geneally, the equlbum model povdes good esults at tempeatues above 1000 C, and t fals at the eacto tempeatues below 800 C [7-8]. In ths wok, the equlbum model was combned wth some empcal coelatons based on laboatoy expements to pedct the cabon conveson, eacto tempeatue, gas composton, gas heatng value and the gas ta content unde dffeent types and amounts of gasfcaton agents. The nput data fo the model was detemned by the chaactesaton of sngle components of RDF such as pape, polyethylene fols, had plastcs and textles. GeoScence Engneeng Volume LXII (2016), No. 1

2 2 RAW MATERIAL The RDF samples used n ths study wee povded by a ecyclng company based n Slovaka. The composton of the RDF was estmated by hand sepaaton of a epesentatve sample of RDF nto ts basc mateal categoes. The contents of ndvdual components of RDF ae shown n Table 1. The sample of RDF wth the composton shown n Table 1 was homogensed by goundng n a cyogenc mll. In addton, homogensed samples of ndvdual mateal categoes of RDF wee pepaed. Table 1: Composton of RDF samples used Component Mateal w [kg/kg] Pape Fol Plastcs Whte pape, ecycled pape LDPE, DPE Rgd plastcs, polystyene, polyuethane Textle Polyamde, polyeste, cotton, wool Usng smultaneous themogavmetc (TG)/dffeental scannng calometc (DSC) measuements (Netzsch STA 409 PC Luxx, selb, Gemany), a poxmate analyss of ndvdual components of RDF was done. The expemental condtons wee met at the lnea heatng ate of 5 K mn -1 n the ntogen flow of 60 ml h -1. The samples wee heated fom 293 K to 1073 K; at ths tempeatue, they wee mantaned fo aound 30 mn and then combusted by enteng the oxygen to the system. Samples of ndvdual components of RDF, of aound 20 mg, wee used n the TG/DSC measuements. The dy bass elemental composton of ndvdual components of RDF was estmated by a Vao Maco Cube ELEMENTAR elemental analyze. A CNS (Cabon, ydogen, Ntogen, Sulphu) module wth the combuston tube tempeatue of 1423 K, and the educton tube tempeatue of 1073 K was used. The module dd not enable the detemnaton of the Cl content. The mass of the sample was aound 1 g. The esults of both poxmate and elemental analyses ae gven n Table 2. Component Table 2: Poxmate and elemental compostons of RDF and RDF components w [kg/kg] Mostue VM FC AS C N S O* Pape Fol Plastcs Textle RDF *-calculated to 100 %, VM- volatle matte, FC- fxed cabon Based on the elemental composton of RDF shown n Table 2 and the molecula mass of elements, the summay fomula of RDF (dy ash fee) n the fom of C a boc N d Se was poposed. The values of a, b, c, d, and e ae gven n Table 3. Table 3: Dy ash- fee fomula of RDF C N S O* a b c d E The fnal molecula fomula of the RDF used s C O N S and ts molecula mass based on one cabon s g mol -1. GeoScence Engneeng Volume LXII (2016), No. 1

3 3 GASIFICATION MODEL Let us assume that: steady state flow s consdeed nsde the gasfe no tempeatue and concentaton gadent exst nsde the eacto the esdence tme s long enough to each the complete decomposton of RDF, and the uneacted pat of RDF s only cabon only the majo speces ae consdeed n the poduct gases,.e. CO, CO 2, 2, C 4, 2 O, N 3, 2 S, N 2, and Ta Ntogen and sulphu pesented n the RDF s conveted only to N 3 and 2 S. Then the global mateal balance of RDF gasfcaton can be wtten as: C O N S x O x O x CO x CO x x C x O b c d e x N x S x C O N S b1 c1 d1 e1 whee, x 1 and x 2 ae the numbes of moles of oxygen and steam used fo each mole of the RDF and, x 3, x 4, x 5, x 6, x 7, x 8, x 9, and x 10 ae the numbes of moles of each coespondng speces, obtaned fom each mole of C O N S s an empc fomula of ta that was estmated n the RDF based on a sngle atom of cabon. b1 c1 d1 e1 same way as that of the RDF usng the esults of the GC/MS analyss of bomass ta [9]. The followng lnea dependence between the ta content and the tempeatue based on bomass gasfcaton expements n [9] was assumed: y ( T ) whee y epesents the amount of ta n mg of ta pe g of RDF and T s the tempeatue n K. The mass balance of ndvdual elements povdes fve algebac equatons. Togethe wth the total mass balance, t makes sx equatons; howeve, to estmate all ten unknown stochometc coeffcents, at least ten equatons ae equed. The equlbum constant of fou gasfcaton eactons should be added to the model. The followng eactons wee selected fo ths study: Boudouad eacton C+CO 2 =2CO Wate gas shft eacton CO+ 2 O=CO 2 + 2, Methanaton eacton C+2 2 =C 4 Steam gasfcaton of cabon C+ 2 O=CO+ 2 The elaton between the equlbum constant and the equlbum composton of the poducts of ndvdual eactons s gven by: K a P 0 P whee K a s the equlbum constant, P 0 the pessue at whch K a s defned [Pa], P the system pessue [Pa], the stochometc coeffcent of component, φ the fugacty coeffcent of component, and x the mola facton of component. The value of equlbum constant s calculated at constant tempeatue and pessue usng the standad state Gbbs eacton enegy : G K a e RT G x (2) whee K a s the equlbum constant at K, Δ G the Gbbs enegy at the standad state [J mol -1 ], R the gas constant (R = J mol -1 K -1 ), and T s the tempeatue [K]. The fugacty coeffcents wee assumed to be equal to 1 n ths wok. The standad eacton Gbbs enegy was calculated based on the standad eacton enthalpy, Δ [J.mol -1 ], and the standad eacton entopy, Δ S [J mol -1 K -1 ] as: G T S (3) 39 (1) GeoScence Engneeng Volume LXII (2016), No. 1

4 The values of Δ and Δ S wee calculated usng the standad eacton enthalpy and the entopy of fomaton: 40 f (4) S S (5) whee s the stochometc coeffcent, Δ the standad enthalpy of fomaton [J mol -1 ], and Δ S the standad entopy of fomaton [J mol -1 K -1 ]. The eacton enthalpy and the eacton entopy at the tempeatue of the system can be calculated by: cp ( T ) (6) T S S cp ln whee Δ s the eacton enthalpy at tempeatue T [J mol -1 ], Δ S the eacton entopy at tempeatue T [J mol -1 K -1 ], and c p the aveage mola heat capacty of component [J mol -1 K -1 ]. In the gasfcaton eacto, the eacton tempeatue s obtaned by patal oxdaton of the feed. The global enthalpy balance of the eacto s: whee Q Q RDF O2( a ) steam R gas ash C loss Q s the heat of eacton [J], R oxygen and a, espectvely [J], the enthalpy of ash [J], eacto [J]. f RDF the enthalpy of RDF feed [J], O2( a) ae the enthalpy of steam s the enthalpy of wate steam [J], gas the enthalpy of gas [J], ash C the enthalpy of uneacted cabon [J], and Q loss epesents the heat losses fom the The heat of eacton can be calculated as the dffeence between the heatng values of eactants and poducts: (9) Q m wq ( ) n R RDF c whee m RDF s the mass flow of RDF feed [kg], n s the mole flow of component n the poducts [kmol], w the mass facton of component n the feed (pape, fol, plastcs, textles) [-], Q the lowe heatng value of component n the feed (pape, fol, plastcs, textles) [J kg -1 ], and c s the heat of combuston of component n the poducts [J kmol -1 ] If the RDF feed and oxygen (a) ae fed to the eacto at the same tempeatue whch s selected as the efeence tempeatue, T ef, then =0, O2( a) = 0 and RDF (10) ( T T )( n c m c m c m c ) gas ash C steam ef p C pc ash pash steam psteam whee m ash s the mass flow of ash [kg], m c the mass flow of emanng cabon [kg], m steam the mass flow of steam [kg], c the specfc heat capacty of ash [J kg -1 K -1 ], c pc the specfc heat capacty of emanng pash cabon [J kg -1 K -1 ], and cpsteam s the specfc heat capacty of steam [J kg -1 K -1 ]. Substtutng the equatons (10) and (9) nto the equaton (8) fo the tempeatue of poducts, the followng esults wee obtaned: T T ef m wq ( ) n Q RDF c loss nc p mcc pc mashc pash msteamcsteam (11) (7) (8) GeoScence Engneeng Volume LXII (2016), No. 1

5 X XN2 The used heatng values fo poducts and components ae shown n Table 4. The lowe heatng values of RDF components wee measued expementally [10] and the heat of combuston of the gas components was found n chemcal engneeng lteatue. Table 4: Used heatng values Component Q [J/g] -Δ c [J/mol] Pape Fol Plastcs Textle CO C C RESULTS AND DISCUSSION The model descbed above was solved usng MS EXCEL n the fst step, then t was mplemented nto the ASPEN Plus smulaton envonment. The gasfcaton wth the followng thee dffeent gasfcaton agents was caed out: gasfcaton of RDF usng a gasfcaton of RDF usng pue oxygen gasfcaton of RDF usng pue oxygen and steam The vaable was the mass ato of the gasfcaton agent (a, oxygen o steam) and the obseved output paametes wee: conveson of RDF, eacto tempeatue, gas composton, and gas hghe heatng value (V). a - Gasfcaton of RDF usng a: Fgue1 shows the composton of poduced gases at dffeent a to RDF mass ato. 2 and CO ae the most mpotant components of the poduced gas; the content of 2 showed a maxmum at m (A) /m (RDF) =2 and that of CO at m (A) /m (RDF) =2.25. The content of methane deceased to a value nea zeo at m (A) /m (RDF) =2.5. oweve, t esults fom Fgue 2 that at these values of m (A) /m (RDF), the adabatc eacto tempeatue s aound 850 K and the conveson of RDF s only 40-50%. A 100 % conveson was acheved at m (A) /m (RDF) =3.2 at the eacto tempeatue of aound 950 K. The gas heatng value was elatvely low, 4.5 MJ N -1 m -3, because of a hgh ntogen content, and t emaned pactcally constant fo m (A) /m (RDF) angng fom 2 to 3.2, when t apdly deceased because of the statng CO oxdaton to CO 2 and a apd ncease of ntogen content as a esult of the deceasng contents of 2 and CO. A low eacto tempeatue can cause a hgh content of ta n the poduced gas. But nceasng the eacto tempeatue eques the oxdaton of 2 and CO and also a apd ncease of ntogen content n the poduced gas whch esults n losng the enegy content of the gas. Based on ths obsevaton, the optmal value of m (A) /m (RDF) fo the gasfcaton of RDF studed n ths wok was Gas Composton X2 XCO XCO2 XC4 XN m(air)/m(rdf) Fgue 1: Gas composton at dffeent m (A) /m (RDF) values usng a as the gasfcaton agent GeoScence Engneeng Volume LXII (2016), No. 1

6 TR/ K CON/ % Mole facton Reacto Tempeatue, Conveson and Gas eatng Value m(air)/m(rdf) Reacto Tempeatue/ K Conveson/ % Fgue 2: Conveson, eacto tempeatue and gas heatng value at dffeent m (A) /m (RDF) values usng a as the gasfcaton agent The ta content of the poduced gas deceased wth the nceasng eacto tempeatue and the a to RDF mass ato. At optmum m (A) /m (RDF) =3.2, the ta content was 280 mg/nm 3, whch s not vey hgh but stll above the lmt value of 50 mg/nm 3 [2] fo gas tubnes. Ta Content/(mg/Nm 3 ) m (a) /m (RDF) Fgue 3: Content of ta n the poduced gas at dffeent m (A) /m (RDF) values usng a as the gasfcaton agent The contents of 2 S and N 3 deceased wth nceasng m (A) /m (RDF) (Fgue 4); the content of 2 S at m (A) /m (RDF) =3.2 was mol %. The combuston of such gas esults n the SO 2 content n the flue gases of aound 50 mg/nm 3, whch s below the EU lmt value fo the combuston of sold fuels (200 mg/nm 3 ). 0,0006 0,0005 0,0004 0,0003 0,0002 0,0001 N3 2S m (a)/ m (RDF) Fgue 4: Content of 2 S and N 3 of the poduced gas at dffeent m (A) /m (RDF) values usng a as the gasfcaton agent GeoScence Engneeng Volume LXII (2016), No. 1

7 X TR/ K CON/ % 43 b - Gasfcaton of RDF usng oxygen: The oxygen blown gasfcaton of RDF enables the poducton of gas wth a fold hghe heatng value. oweve, the gasfcaton plant eques an a sepaaton unt. The complete conveson of RDF was acheved at m (O2) /m (RDF) =0.65; the adabatc eacto tempeatue unde these condtons was aound 1100 K and the gas heatng value was appox. 10 MJ/Nm 3. The content of 2 showed ts maxmum at m (O2) /m (RDF) =0.38 (50 mol.%), but the conveson at ths pont was only 50 %. The content of CO showed ts maxmum at m (O2) /m (RDF) =1.05, (55 mol.%), but the 2 content at ths pont was 25 mol.% and that of CO 2 was 18 mol.%, whle ts mnmum value at m (O2) /m (RDF) =0.75 was 10 mol.%. Based on these obsevatons (Fgues 5 and 6), the optmal value of m (O2) /m (RDF) fo the gasfcaton of the RDF studed n ths wok was The equlbum model based calculated gas compostons publshed by othe authos showed a smla tend. oweve, the expementally measued data showed lowe 2 and hghe CO 2 contents of the gas. These dffeences ae sgnfcant patculaly at tempeatues below 800 C Reacto Tempeatue, Conveson and Gas eatng Value Tempeatue/ K Conveson/ % m(o2)/m(rdf) Fgue 5: Conveson, eacto tempeatue and gas heatng value at dffeent m (O2) /m (RDF) values usng oxygen as the gasfcaton agent Gas Composton X2 XCO XCO2 XC Fgue 6: Gas composton at dffeent m (O2) /m (RDF) values usng oxygen as the gasfcaton agent c - Gasfcaton of RDF usng oxygen and steam: m(o2)/m(rdf) The addton of steam to the gasfcaton eacto enables takng cae of the wate-gas shft eacton and the steam gasfcaton eacton of cabon. As a esult of these eactons, a hghe content of 2 and CO 2 and a lowe content of CO n the poduced gas wee obtaned. At constant m (O2) /m (RDF) =0.67, the gas composton, eacto tempeatue and the gas heatng value at dffeent m (steam) /m (RDF) values wee detemned. Peheated steam wth the tempeatue of 600 K and the pessue of 12 ba was consdeed. It was found that by nceasng the mass ato of steam to RDF fom zeo to 0.55, the content of 2 nceased fom 44 mol.% to 51 mol.% and that of CO 2 nceased fom 10 mol. % to 22 mol.%. oweve, the content of CO deceased fom 41 mol.% to 23 mol.%, the eacto tempeatue deceased by 40 K and the gas heatng value deceased fom 10.1 MJ/Nm 3 to 8.7 MJ/Nm 3. The effect of steam addton to a gasfcaton eacto at m (O2) /m (RDF) =0.67 s shown n Fgue 7. GeoScence Engneeng Volume LXII (2016), No. 1

8 TR/ K X Effect of Steam Reacto Tempeatue/ K X2 XCO XCO2 XC m(steam)/m(rdf) Fgue 7: Effect of steam addton to gasfcaton eacto at m (O2) /m (RDF) = CONCLUSION The 100% RDF conveson was acheved n the gasfcaton wth a at m (a) /m (RDF) = 3.2. oweve, the gas heatng value was only 4.4 MJ/Nm 3, and the eacto tempeatue was 950 K. The poduced gas contaned 19 mol.% of 2, 16.5 mol.% of CO, 7.6 mol.% of CO 2, N 2, and a mno amount of othe components. The contents of 2 S and N 3 dd not exceed the EU lmts fo SO 2 and NOx caused by the combuston of these components. The gasfcaton of RDF usng oxygen enables the poducton of gas wth the heatng value of aound 10 MJ/Nm 3 at m (O2)/ m (RDF) =0.67. Unde these condtons, the content of 2 was 44 mol %, that of CO was 41 mol %, and that of CO 2 was aound 10 mol.%; the content of othe components ncludng methane was 5 mol.%. By nceasng the m (steam) /m (RDF) ato, the contents of 2 and CO 2 nceased. oweve, the content of CO, the eacto tempeatue and the gas heatng value deceased. ACKNOWLEDGEMENT Ths wok was suppoted by the Gant VEGA No. 1/0757/13 fom the Slovak Scentfc Gant Agency. REFERENCES [1] BELGIORNO V., DE FEO G., DELLA ROCCA C., NAPOLI R.M.A. Enegy fom gasfcaton of sold wastes. Waste Management. 2003, 23(1), [2] IGMAN C., VAN DER BURGT M. Gasfcaton. Second edton. Gulf Pofessonal Publshng Elseve: Oxfod, UK, p. ISBN [3] MATERAZZI M., LETTIERI P., MAZZEI L., TAYLOR R, CAPMAN C. Themodynamc modellng and evaluaton of a two-stage themal pocess fo waste gasfcaton, Fuel. 2013, 108, [4] BARBA D., PRISCIANDARO M., SALLADINI A., DI CELSO M. The Gbbs Fee Enegy Gadent Method fo RDF gasfcaton modellng. Fuel. 2011, 90, [5] JARUNGTAMMACOTE S., DUTTA A. Equlbum modelng of gasfcaton: Gbbs fee enegy mnmzaton appoach and ts applcaton to spouted bed and spout-flud bed gasfes. Enegy Conveson and Management. 2008, 49, [6] LI X., GRACE J.R., WATKINSON A.P., LIM C.J., ERGÜDENLER, A. Equlbum modelng of gasfcaton: a fee enegy mnmzaton appoach and ts applcaton to a cculatng fludzed bed coal gasfe. Fuel, 2001, 80, [7] MATERAZZI M., LETTIERI P., TAYLOR R., CAPMAN C. Pefomance analyss of RDF gasfcaton n a two stage fludzed bed plasma pocess. Waste Management. 2016, 40, [8] ALTAFINI C.R.,WANDER P.R., BARRETO R.M. Pedcton of the wokng paametes of a wood waste gasfe though an equlbum model. Enegy Conveson and Management. 2003, 44, [9] LBOCKÁ D. Znžovane obsahu dechtu v pocesnom plyne p splyňovaní lgnocelulózovej bomasy- vplyv pocesných paametov. Batslava, Dploma thess. FCPT STU, 2015, 81 p. [10] MRVA M. Pyolýza a splyňovane tuhého komunálneho odpadu. Batslava, Dploma thess. FCPT STU, 2014, 63 p. GeoScence Engneeng Volume LXII (2016), No. 1